Nanostructured solid electrolytes and devices

ABSTRACT

Ion conducting solid electrolytes are constructed from nanoscale precursor material. Nanocrystalline powders are pressed into disc structures and sintered to the appropriate degree of densification. Metallic material is mixed with 0 to 65 vol % nanostructured electrolyte powders to form a cermet mix and then coated on each side of the disc and fitted with electrical leads. The electrical conductivity of a Ag/YSZ/Ag cell so assembled exhibited about an order of magnitude enhancement in oxygen ion conductivity. As an oxygen-sensing element in a standard O 2 /Ag/YSZ/Ag/N 2  set up, the nanocrystalline YSZ element exhibited commercially significant oxygen ion conductivity at low temperatures. The invention can be utilized to prepare nanostructured ion conducting solid electrolytes for a wide range of applications, including sensors, oxygen pumps, fuel cells, batteries, electrosynthesis reactors and catalytic membranes.

RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 08/739,257, filedOct. 30, 1996, now U.S. Pat. No. 5,905,000, which is acontinuation-in-part of U.S. Ser. No. 08/730,661, entitled “PassiveElectronic Components from Nano-Precision Engineered Materials,” filedon Oct. 11, 1996, now U.S. Pat No. 5,952,040, which is acontinuation-in-part of U.S. Ser. No. 08/706,819, filed Sep. 3, 1996entitled “Integrated Thermal Process and Apparatus for the ContinuousSynthesis of Nanoscale Powders,” now issued as U.S. Pat. No. 5,851,507on Dec. 22, 1998, and U.S. Ser. No. 08/707,341, entitled “Boundary LayerJoule-Thompson Nozzle for Thermal Quenching of High Temperature Vapors,”filed concurrently on Sep. 3, 1996, now issued as U.S. Pat. No.5,788,738 on Aug. 4, 1998. These applications and patents are allcommonly owned with the present application, and are all incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains in general to ion conductors and to processesfor the synthesis of ion conducting solid electrolytes. In particular,the invention relates to the use of nanoscale powders for thepreparation of nanostructured oxygen ion conducting electrolytes.

2. Description of the Prior Art

Solid electrolytes are materials through which ion species can migratewith low energy barriers. Table 1 outlines some examples ofion-conducting structures, representative materials, and the ionsconducted. These materials are of critical commercial importance toelectrochemical devices, components and processes. Illustrativeapplications include sensors, batteries, fuel cells, ion pumps, membranereactors, catalysis, and metallurgy.

REPRESENTATIVE MATERIALS ION CONDUCTED Stabilized ZrO₂ System,Stabilized Bi₂O₃ O²⁻ System, Ceria, Perovskites Beta-Alumina, NASICONSystems Na⁺ AgI, RbAg₄I₅ Ag⁺ Rb₄Cu₁₆I₇Cl₁₃ Cu⁻ Li₃N, Li₂S—SiS₂—Li₃PO₄System, Organic Li⁺ Polymer Systems, LISICON Systems

As a specific example, stabilized zirconia is a known conductor ofoxygen ions. Accordingly, its properties are utilized in various fieldsof technology, such as in oxygen sensors for fuel-air ratio optimizationof automobiles and furnaces, in oxygen pumps for solid state oxygenseparation, in solid-oxide fuel cells for noiseless and clean powergeneration from chemical energy, and in catalytic membrane reactors.

The oxygen-ion conduction properties of stabilized zirconia used in atypical oxygen sensor are well understood based on electrochemical-celltheory. When placed between two compartments containing a reference gasand an analyte oxygen gas at different partial pressures, stabilizedzirconia functions both as a partition between the two compartments andas an electrochemical-cell electrolyte. Under ideal conditions, theopen-circuit emf (E₀) of the cell is given by the known Nernst equation:$\begin{matrix}{{E_{0} = {\frac{RT}{4F}{\ln \left( \frac{{PO}_{2}^{Ref}}{{PO}_{2}} \right)}}},} & (1)\end{matrix}$

where T is the absolute temperature of the cell; PO₂ ^(Ref), and PO₂ arethe partial pressures of oxygen in the reference and analytecompartments, respectively; R is the universal gas constant; and F isFaraday's number.

According to this equation, any difference in partial pressure of theoxygen across the two faces of the oxygen-conducting electrolytegenerates an electromotive force that depends on the temperature andpartial-pressure ratio of the oxygen in the two compartments of thesystem. In order to generate Nernstian response in sufficiently shorttimes, the temperature of stabilized ZrO₂ needs to be high (above 700°C.), which results in relatively high power requirements and inincreased equipment mass and size, need for insulation, and attendantsealing problems. These considerations often produce unsatisfactoryperformance or affect the commercial viability of products based onstabilized ZrO₂ technology.

The inherent reasons for the high-temperature requirement and thecorresponding performance problems of present-day oxygen ion conductingelectrolyte based devices can be traced to the reaction mechanism of thecell and the microstructure of the sites where the reaction occurs.Referring to FIG. 1, a schematic drawing of a ZrO₂ sensor cell 10 isillustrated, where the stabilized zirconia is modeled as a solidelectrolyte membrane 12 between a first compartment 14, containing areference oxygen atmosphere at a predetermined partial pressure PO₂^(Ref), and another compartment 16 containing an analyte gas with oxygenat a different partial pressure PO₂. The two sides of the stabilizedzirconia non-porous solid electrolyte 12 are coupled through an externalcircuit connecting an anode 18 and a cathode 20 made of porous metal,such as silver. The anode 18 is the cell electrode at which chemicaloxidation occurs and the electrons released by the oxidation reactionflow from it through the external circuit to the cathode. The cathode 20is the cell electrode at which chemical reduction occurs. The cellelectrolyte 12 completes the electrical circuit of the system byallowing a flow of negative ions O²⁻ between the two electrodes. Avoltmeter 22 is provided to measure the emf created by the redoxreactions occurring at the interfaces of the electrolyte with the twooxygen atmospheres.

Thus, the key redox reaction of the cell occurs at the points where themetal electrode, the electrolyte and the gas meet (illustrated in theinset of FIG. 1 as the “triple point” 24 ). At each such site on thesurface of the electrolyte 12, the redox reaction is as follows:

O₂.(gas)+4e⁻→2O²⁻.  (2)

Since the reaction and the electrochemical performance of the sensordepend on the redox kinetics, the cell's performance is a strongfunction of the concentration of triple points. In other words, anelectrode/electrolyte/electrode cell with as many triple points aspossible is highly desirable [see Madou, Marc and M. Morrison, ChemicalSensing with Solid State Devices, Academic Press, Boston (1989)]. In thecase of an oxygen cell with a ZrO₂ solid membrane and silver electrodes,this requirement corresponds to maximizing the triple points on eachside of the PO₂.Ag′/ZrO₂/Ag″PO₂ ^(Ref) system.

Another cause of poor performance of oxygen-sensor cells can beexplained with the help of complex-impedance analysis. Referring toFIGS. 2a and 2 b, a complex impedance diagram for a ZrO₂ sensor isshown, where the impedances of the bulk, grain boundary and electrodeare illustrated in series to reflect their contribution to the ionicconduction at each triple point. It has been shown that the conductiveperformance of electrolytes at temperatures below 500° C. is controlledby the grain boundary contribution to the overall impedance. Thus, forsignificant improvements of the conductivity at low temperatures, it isnecessary to significantly minimize the grain-boundary (interface)resistance.

In summary, oxygen ion conducting devices based on stabilized-zirconiaelectrolyte have two problems that can be traced to materiallimitations. First, the electrolytes have high impedance; second, theconcentration of triple points is relatively low. These problems arecommon to solid oxygen-conducting electrolytes in particular and solidelectrolytes in general, and any improvement in these materialcharacteristics would constitute a significant technological stepforward. The present invention provides a novel approach that greatlyimproves these aspects of ion conducting solid electrolytes.

SUMMARY OF THE INVENTION

One of the objectives of this invention is to enhance theion-conductivity of solid electrolytes by preparing nanostructured solidelectrolytes.

Another objective is to reduce the electrolyte thickness with the use ofnanostructured precursors of solid electrolytes.

A further objective is to enhance the concentration of triple points inthe ion conducting devices by using nanostructured precursors andmaterials.

Yet another objective of the invention is to utilize the uniqueproperties of size confinement in solid electrolyte and electrode grainswhen the domain is confined to less than 100 nanometers.

Another objective of this invention is an oxygen-conducting electrolytematerial with low-impedance oxygen conducting characteristics.

Another objective of the invention is an oxygen ion conducting devicewith a very high density of triple points.

Another goal is a process and materials that reduce the cost ofmanufacture of products that incorporate oxygen-ion conductors.

Yet another goal is a process and materials that reduce the cost ofoperation of products that incorporate oxygen-ion conductors.

Finally, another goal is a process that can be readily incorporated withconventional methods for manufacturing products containingion-conducting electrolytes.

Various other purposes and advantages of the invention will become clearfrom its description in the specification that follows and from thenovel features particularly pointed out in the appended claims.Therefore, to the accomplishment of the objectives described above, thisinvention consists of the features hereinafter illustrated in thedrawings, fully described in the detailed description of the preferredembodiments and particularly pointed out in the claims. However, suchdrawings and description disclose only some of the various ways in whichthe invention may be practiced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a ZrO₂ solid electrochemical cell wherethe stabilized zirconia is modeled as a solid electrolyte membranesandwiched between a first compartment containing a reference oxygenatmosphere at a predetermined partial pressure and another compartmentcontaining an analyte gas with oxygen at a different partial pressure.

FIGS. 2a and 2 b are a complex impedance diagram for a ZrO₂ sensor,where the impedances of the bulk, grain boundary and electrode areillustrated to reflect their contribution to the ionic conduction ateach triple point shown in FIG. 1.

FIG. 3 is an X-ray diffraction pattern of the nanoscale yttriastabilized zirconia precursor used to form an electrolyte membraneaccording to the invention.

FIG. 4 is a transmission electron microscope image of the nanoscaleyttria stabilized zirconia nanopowder produced in Example 1.

FIG. 5 is a graph of total conductivity versus temperature of 9-YFSZnanozirconia prepared by the process of Example 1 and micron-based YFSZmaterial.

FIG. 6 is a voltage-versus-temperature graph of a nanostructured oxygensensor manufactured with the material prepared in Example 1. Thesymmetrical response about the abscissa relates to switching the gasesfrom one face of the sensor to the other.

FIG. 7 is a flow chart of the process for preparing a yttria stabilizedbismuth oxide (YSB) nanopowder from nitrates by a solution method.

FIG. 8 is an X-ray diffraction pattern of the nanostructured YSB productof Example 2.

FIG. 9 is a transmission electron microscope image of the nanostructuredYSB produced in Example 2.

FIG. 10 is a scanning electron microscope (SEM) image of the compositestructure resulting from the densification of the Ag phase inhibited byan ion electrolyte material phase of Example 3.

FIG. 11 illustrates Nyquist plots of impedance spectra of cells with thenanocomposite electrodes of Example 3 in comparison with a cell withpure Ag electrodes.

FIG. 12 shows electrode resistances of a pure Ag electrode andnanocomposite electrodes as a function of temperature, as determinedfrom the impedance spectra measured in air.

FIG. 13 is the voltage response in oxygen of the sensor of Example 3 asa function of temperature in comparison with that of a conventional YSZsensor.

FIG. 14 shows a comparison of the ionic conductivity of nanostructuredYSB electrolyte with YSZ electrolyte.

FIG. 15 is a flow diagram of the steps of deposition over a supportingsubstrate according to known vapor deposition processes applicable tothe manufacture of nanostructured electrolytes according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

This invention is based on the recognition that the ion conductivity ofpolycrystalline solid electrolytes at moderate and near-ambienttemperatures is mainly controlled by the conductivity of grain boundaryand the concentration of triple points. The invention further notes thatthe engineering of grain boundary resistance and triple points in solidelectrolyte devices is limited by the electrolyte thickness andelectrode characteristics, respectively, which in turn depend on grainsize of the precursors and the material used in the manufacture ofelectrolytes and electrodes for solid ion conductors in general, andsolid oxide oxygen sensors, solid oxide oxygen pumps and solid oxidefuel cells in particular. These limitations constitute an inherentobstacle to achieve significant technological improvements.

The finest powders currently available for commercial use consist ofparticles with sizes in the order of several microns. For example, theYSZ powders that are presently used to produce oxygen sensors have anaverage grain size of about 1 to 3 microns. Since the number of triplepoints occurring within a given area at the interface with the oxygenatmosphere is necessarily limited by the number of electrode grainsdistributed within that area, the grain size in the electrode is veryimportant for maximizing redox-reaction sites. Similarly, since we knowthat the impedance of the system is reduced by electrolyte thickness, itfollows that thinner electrolytes produced from smaller grains wouldproduce lower impedance. Accordingly, the heart of this inventionconsists of using nanosize materials in the manufacture of electrolytesfor these applications.

The current inability to improve the performance of solid ion conductorsis a result of the inability of prior-art processes to economicallyreduce powder size of precursor materials beyond the micron-size range.Accordingly, the present invention is based on the work disclosed incommonly-owned U.S. Pat. Nos. 5,851,507 and 5,788,738, which provide aviable vehicle for manufacturing nanoscale powders suitable for thepresent invention. Material having with physical properties as producedby the process and apparatus described therein is a necessary ingredientfor practicing this invention on a commercial scale.

As defined in the art, submicron powders are materials having averagegrain size below 1 micrometer. Of critical interest for this inventionare nanoscale powders and nanostructured layers of ceramics andelectrodes. Nanoscale powders (nanopowders) are submicron powders withaverage grain size less than 100 nanometers (preferably with a standarddeviation of less than about 25 nm) and with a significant fraction ofinterfacial atoms. Accordingly, reference to nanoscale powders in thisdisclosure is intended to refer to powders with those characteristics.

Submicron layers are layers having thickness less than 1 micrometer. Ofparticular interest to this invention are nanostructured layers whichare defined specifically as layers with thickness, or microstructure, orboth, confined to a size less than property confinement size (positivelyless than 1 micron, preferably below 100 nm). Accordingly, reference tonanostructured layers in this disclosure is also intended to refer tolayers with those characteristics.

As discussed in the copending applications, it is known that withinthese size ranges a variety of confinement effects occur thatdramatically change the properties of the material. The idea of thisinvention then is to build ion conducting solid electrolytes frompowders whose grain size has been confined to dimensions less than 100nanometers. The size confinement effects in nanometer scale can confinefundamental processes to band-gap and quantum confined states which inturn can dramatically change the properties and performance of theresulting solid electrolyte. This insight can be implemented as devicesprepared with one dimensional quantum dot and nanocluster composite withthe dot size less than 100 nm (preferably less than 10 nm), as quantumwires with diameter less than 100 nm (preferably less than 10 nm), asquantized and nanoscale films with film thickness less than 100 nm, asnanostructured layers and pellets with microstructure less than 100 nm,and as a combination of these. In summary, another aspect of theinvention concerns the preparation of solid electrolyte and electrodesthat are nanostructured.

Nanostructured ion conducting electrolytes prepared from nanostructuredmaterials have grain sizes spatially confined to less than 100nanometers, a significant fraction (20-60%) of their atoms isinterfacial, and exceptional interactions occur between the constituentdomains. Therefore, nanostructured oxygen-conducting electrolytes can beexpected to have very high concentrations of interface area which canassist rapid and low-temperature densification of ion conductingelectrolytes. The nanoscale powder can also enable dramatic reduction inlayer thicknesses as discussed in co-pending applications. Furthermore,since nanostructure provides higher density of surface area, the densityof triple points at the electrolyte-electrode-gas interface can also besignificantly enhanced using nanostructured-electrolyte/electrodeinteractions. Given low resistance and high triple-point concentration,nanostructured electrolytes and electrodes can be used to achieve higherion conductivity and electrochemical activity. This is of particularinterest when an ion conducting device has to operate at near ambienttemperatures. This general design principle is applicable to all, solidion conductors based on ion defect structure, two dimensional layeredstructure, three dimensional network structure, vitreous structure,α-AgI type structure, and composites prepared using these structures.Illustrative examples include, without limitation, oxide ion conductorssuch as stabilized zirconia, stabilized ceria, stabilized bismuth oxide,perovskites, LISICON, NASICON, and β-alumina.

The following examples illustrate different ways of reducing the presentinvention to practice.

EXAMPLE 1

A stock solution was prepared from ZrOCl₂.8H₂O and 9 mol % Y₂O₃ inwater, and diluted with denatured ethanol. The solution was chilled to0° C. and then slowly added to a continuously stirred basic solution ofammonium hydroxide that was also maintained at 0° C. Precipitation ofwhite precursor powder was instantaneous. The precipitate solution wassuction filtered, and the gelatinous filter cake was washed in denaturedethanol three times. The loose powder so generated was dried quicklywith mild heating at 100° C. to remove water and ethanol, and calcinedto 500° C. in air to form nanocrystallites with grain size of about 5.8nm, standard deviation of 1.1 nm. This precursor material consisting of9 mole-percent yttria stabilized zirconia (YSZ) nanoscale powders wasexamined using an X-ray diffractometer (XRD). A typical XRD pattern forthe 9 mole powders so produced is illustrated in FIG. 3, which showsthat the ZrO₂ is stabilized cubic phase. In order to determine theaverage particle size of the powders, the widths of strong, low orderpeaks of XRD pattern were analyzed using Scherrer's method. The averageparticle size of the powders according to this analysis was found to beabout 4.5 nanometers. The particle size was also verified bytransmission electron microscopy (TEM). FIG. 4 is the resulting TEMimage suggesting a particle size of 5.8 nanometers.

The nanoscale 9 mole % yttria stabilized cubic zirconia powders werepressed into 3 mm diameter discs (0.15 gram weight) and sintered to highdensities (preferably more than 90% of theoretical density formechanical strength, over 95% being preferred). The sample disks weresintered at low temperatures (1,150 to 1,250° C., yielding more than 95%density) and for short duration (6 to 24 hours) to minimize graingrowth. We found that YSZ nanopowders readily sintered to fulltheoretical densities at about 1,200° C. in 17 hours, which representsignificantly milder and less expensive processing conditions thanpresently necessary. Careful control of the sintering profile and timecan further reduce the sintering temperature and time. The cylindricaldiscs were examined under XRD and the post-sintered mean grain size byScherrer analysis was found to be about 83 nm, confirming that the discswere nanostructured. The two ends of the cylindrical discs so producedwere then coated with a cermet paste consisting of a mix of silver andnanoscale yttria stabilized zirconia powder (about a 50—50 wt % mix,corresponding to a 35Ag-65YSZ vol % mix). Then platinum leads wereattached to the cermet layer.

The samples were placed in a furnace and their impedance was measured inair as a function of increasing temperature with a computerizedimpedance analyzer. A standard 40 mV AC bias and frequency sweep rangeof 5 Hz to 13 MHz were used. As illustrated in FIG. 5, the results soobtained suggest that nanostructured oxygen-conducting electrolytes,referenced by n, exhibit almost an order of magnitude higher oxygen-ionconductivities at lower temperatures when compared with base-lineelectrolytes, referenced by μ (i.e., conventional micron-powder basedoxygen-conducting electrolytes). It is noted that neither the base-linenor the nanostructured electrolytes represent optimal performance. Foradditional electrochemical and electrocatalytic performance evaluation,the Ag/YSZ/Ag cell was tested as a sensor and oxygen pump. Forsensor/fuel-cell experiments, oxygen containing gas was passed over oneface of the sensor and nitrogen was passed over the other face of thesensor. The emf response as a function of temperature was measured. Asshown in FIG. 6, the results indicate that the sensor signal for eachgas combination is linear with temperature, confirming a Nernst-typebehavior. For electrosynthetic oxygen generation and pumpingapplications, CO₂ was passed over one face while nitrogen was passedover the other face. FIG. 6 shows that the oxygen-conducting electrolyteexhibited oxygen pumping properties at low temperatures.

EXAMPLE 2

Bismuth nitrate (Bi(NO₃)₃.5H₂O) and yttrium nitrate (Y(NO₃).6H₂O), wereused as precursors for preparing nanosized yttria stabilized bismuthoxide (YSB) powder via solution co-precipitation. FIG. 7 shows a flowchart of the co-precipitation processing steps used in this example.After precipitation, the precipitate solution was suction filtered, andthe gelatinous filter cake was washed in acetone to minimizeagglomeration of ultrafine powder due to hydrogen bonding. The loosepowder so generated was next dried with mild heating to remove water andacetone. Then the powder was calcined in air at 500° C. for 2 hours. XRDshowed that calcine schedule resulted in a single cubic YSB phase (seeFIG. 8). The volume averaged crystallite size of the powder fired at500° C. was determined to be 12.5 nm by analyzing the broadening of the(111) diffraction peak and applying Scherrer's formula. The YSBnanopowder was characterized in terms of morphology and particle size bytransmission electron microscopy (TEM). FIG. 9 is a TEM image of thenanopowder prepared. From the image, the average particle size wasestimated to be about 15 nm, which is in good agreement with the resultobtained from XRD analysis.

The nanopowders were uniaxially pressed at 50,000 psi into green pelletsof 12.5 mm in diameter and 1 mm in thickness. The pressing processconsisted of initially lubricating the die with a die lube, followed bythe weighing of an appropriate amount of powder, inserting the powder inthe die, uniaxially pressing to the desired pressure, holding at thatpressure for 30 seconds, and then slowly releasing the pressure over 15seconds. Subsequently, the pellet was forced out from the die. No binderwas added for the forming process. It was found that from the nanopowderthe electrolytes can be sintered with greater than 96% of theoreticaldensity at temperatures ranging from 850 to 950° C. In contrast, YSBelectrolytes made from micron-sized powder are typically sintered attemperatures greater than 1,000° C. It is known that the primary drivingforce for densification of ceramics is the reduction of free surfacearea at high temperatures. The very small size of the YSB nanopowder,therefore, has a very large driving force for densification; thus, therequired sintering temperature can be significantly reduced relative tocommonly used micron-sized powders. This is an important manufacturingadvantage of this invention.

The concept of the invention is also applicable to improve theperformance of electrodes for ion conducting materials. These electrodesshould have high electrical conductivities, high catalytic activities,adequate porosity for gas transport, good compatibility with theelectrolyte, and long-term stability. In order to achieve high catalyticactivities, it is preferred that the electrode be highly porous, so thatit retains a large number of active sites for electrochemical reactions,i.e., the triple points. Ag has been studied as an electrode materialbecause it is known to have high electrical conductivity and highcatalytic activity for oxygen reduction and evolution. However, pure Agelectrodes readily densify during processing and operation, resulting ina dense electrode with little porosity. In order to reduce the electroderesistance, the teachings of this invention were used to preparenanocomposite electrodes from Ag and nanostructured powders of the ionelectrolyte material.

EXAMPLE 3

YSB electrolyte pellets, 19 mm in diameter and 0.9 mm in thickness, weresintered from green pellets of 25 mm in diameter and 1.2 mm inthickness. The pellets were ground and polished to a thickness of 0.7 mmto provide a suitable electrolyte substrate. A separate composite pasteof 79 vol % Ag and 21 vol % YSB was prepared by mixing nanopowder of YSBand unfritted Ag paste (marketed by the Cermalloy Division of HeraeusIncorporated of West Conshohocken, Pa., under Catalog No. C440OUF). Thepaste was printed onto both sides of the pellet to form electrodes, andthen the pellet was fired at 800° C. for 10 minutes to sinter theelectrolyte without densifying it beyond the point necessary to providea robust structure and form a stabilized sensor cell with porouscomposite electrodes (about 18% porosity). Then Ag wire was attached toboth electrodes with a contact of Ag epoxy which was fired at 730° C.for 2 minutes. It is noted that the composite-electrode/electrolytestructure needs to be stabilized by the application of heat, pressure orchemical action, as the particular composition of the compositeconstituents may require or allow, in order to provide a physicallyrobust and stable product.

In the resulting composite electrode structure, we found that thedensification of the Ag phase is inhibited by the ion electrolytematerial phase. The electrodes then retain a porous microstructureduring and after thermally demanding processing and operation, as shownin the SEM image of FIG. 10. The retained porous microstructuresignificantly enhances the performance of the electrodes. In addition,electrodes of this kind have better adhesion to the electrolytes becausethe stress arising from thermal expansion mismatch between the ceramicelectrolyte and the metal electrode is minimized not only by the porous,heterogeneous microstructure of the electrodes but also by tailoring thethermal expansion coefficient of the nanocomposites. We found that ananostructured electrode composite sintered to 50-85 percent of fulltheoretical density (i.e., producing an electrode composite with 15-50percent porosity) is optimal to obtain these advantages of performance.

Further, another important advantage is derived from using nanocompositeelectrodes. If the phases added are ionic conductors or mixedelectronic-ionic conductors, the nanocomposite electrode as a wholeturns out to be a mixed conductor, which allows ambipolar transportwithin the solid phase. FIG. 11 illustrates Nyquist plots of impedancespectra of cells with the nanocomposite electrodes in comparison with acell with pure Ag electrodes. As determined from the impedance spectra,the polarization resistances of the nanocomposite electrodes aresignificantly smaller than that of the pure Ag electrode. As expected,the resistance of the nanocomposite electrode is a function of thecomposition, i.e., the volume fraction of each constituent phase. Thisallows much room for performance optimization by adjusting thecomposition of the composites. The nanocomposite electrode shows anearly 4-fold reduction in electrode resistance as compared to the pureAg electrode. We found that good results are obtained with a mix of 0 toabout 65 vol % electrolyte (35 to 100 vol % metallic electrodematerial), the limit being that a continuum metal phase must exist for aviable porous electrode structure. That is, the amount of electrolytemust not be so great as to cause interruptions in the connectivity ofthe metal phase. At least 5 vol % electrolyte, 21 vol % being preferred,produced good results with different metal/electrolyte combinations. Webelieve that with composition optimization, further reduction inelectrode resistance can be achieved, leading to a significantenhancement in ion conducting device's performnance.

A sensor produced with the structure manufactured in Example 3 wasoperated in the following configuration, using air as the reference gas:

air, 79v%Ag21v%YSB|YSB|79v%Ag21v%YSB, Analyte gas.

The sensor response to the changes in the gas composition and intemperature was monitored by measuring the cell voltage under differentconditions.

Shown in FIG. 13 is the sensor voltage response in oxygen as a functionof temperature in comparison with that of a conventional YSZ sensor.FIG. 13 clearly shows that the response of the YSB sensor follows Nernstbehavior down to 400° C., while the response of the YSZ sensor deviatesfrom Nernst behavior below 500° C. This indicates that the YSBproof-of-concept sensor can be operated at a temperature about 100° C.lower than conventional YSZ sensors.

FIG. 12 further shows electrode resistances of nanocomposite electrodesas a function of temperature, as determined from the impedance spectrameasured in air. Also shown in the figure are the data for pure Agelectrode for comparison. As compared with pure Ag electrode, thenanocomposite electrodes show significantly lower resistances. The ionicconductivity of the nanostructured YSB electrolyte was measured andfound to be over two orders of magnitude higher than that of YSZelectrolyte, as shown in FIG. 14.

The impedance measurements and the data shown in these examplesestablish that nanostructured solid ion electrolytes and electrodes areindeed significantly superior in performance to solid ion electrolytesand electrodes prepared from micron-sized powders. The invention reducedto practice the use of nanostructured ion-conducting electrolytesexhibiting ion conductivity higher than obtained by prior-arttechnology; and it demonstrated the successful fabrication ofion-conducting electrolytes in general, and oxygen-conductingelectrolytes in particular, from materials with grain size less than 100nm for electrochemical, electrosynthesis and electrocatalyticapplications. It is noted that the methods of assembly or deposition ofnanoparticles to form structures according to this invention may varydepending on the particular application. For example, dry particles maybe pressed into a structure of predetermined geometry, as illustrated inExample 1, or deposited over a supporting substrate according to knownvapor deposition processes, as described in U.S. Pat. No. 5,520,040 andillustrated in FIG. 15.

The process of deposition may also be incorporated with the process ofmanufacture of the nanosize particles disclosed in the referencedcopending applications. This method is preferred because it enables thecontinuous fabrication of product from raw material. A thermal reactorsystem is used to produce nanoscale powders by ultra-rapid thermalquench processing of high-temperature vapors through a boundary-layerconverging-diverging nozzle. A gas suspension of the micron-sizedmaterial is continuously fed to a thermal reaction chamber and vaporizedunder conditions that minimize superheating and favor nucleation of theresulting vapor. The high temperature vapor is quenched by passing thevapor stream through the nozzle immediately after the initial nucleationstages, thereby rapidly quenching it through expansion at rates of atleast 1,000° C. per second, preferably greater than 1,000,000° C. persecond, to block the continued growth of the nucleated particles andproduce a nanosize powder suspension of narrow particle-sizedistribution. A gaseous boundary-layer stream is preferably alsoinjected to form a blanket over the internal surface -of the nozzle toprevent vapor condensation in the throat of the nozzle. A receivingsubstrate is placed in the diverging section of the nozzle to receivethe nanoparticles produced in the quenched stream. Thus, ananostructured layer of electrolyte material can be deposited directlyas desired on the particular device being manufactured. As those skilledin the art would readily understand, the precise location of thesubstrate within the nozzle, the residence time, and other operatingparameters could be manipulated to produce the physical structuredesired for a particular application.

Potential applications of the invention include nanostructured solidelectrolyte and electrode based devices for energy storage andgeneration such as, but not limiting to batteries, fuel cells, devicesfor thermodynamic property measurements; electrochemical sensors formonoatomic, diatomic and polyatomic gases such as, but not limiting toatomic oxygen found in atmosphere, diataomic oxygen and ozone sensors;ion sensors; oxygen pumps; solid state chemical pumps; monitors forsteam electrolyzers; measurement of dissolved oxygen in liquid metals;measurement of pH; electrocatalysis, electrosynthesis, catalyticmembrane reactors, and high-temperature kinetic studies. Therefore,while the present invention has been shown and described herein in whatis believed to be the most practical and preferred embodiments, it isrecognized that departures can be made therefrom within the scope of theinvention, which is therefore not to be limited to the details disclosedherein but is to be accorded the full scope of the claims so as toembrace any and all equivalent apparatus and methods.

We claim:
 1. An electrochemical comprising a porous nanocomposite solidelectrode having a first and a second surface, comprising a conductivematerial and nanoscale particles of an electrolyte material and having apore fraction in the range of 15-50 percent, wherein an electricalpotential is developed between the first and second surfaces when thefirst surface is exposed to a reference species and the second surfaceis exposed to an analyte species.
 2. The device of claim 1, wherein thedevice is a gas sensor and the analyte species is selected from thegroup consisting of monatomic oxygen, diatomic oxygen, and ozone.
 3. Anelectrochemical device, comprising a nanostructured electrolyticstructure having an first surface and a second surface, prepared byaggregating nanoscale particles of a ceramic electrolyte material toform the electrolytic structure; and sintering the electrolyticstructure at a temperature sufficient to cause its densification, saidtemperature being less than 1000° C., wherein an electrical potential isdeveloped between the first and second surfaces when the first surfaceis exposed to a reference species and the second surface is exposed toan analyte species.
 4. The device of claim 3, wherein the device is agas sensor and the analyte species is selected from the group consistingof monatomic oxygen, diatomic oxygen, and ozone.
 5. The device of claim1 or 3, wherein the device is a gas sensor and the analyte species is agas.
 6. The device of claim 1 or 3, wherein the device is a chemicalsensor and the analyte species is dissolved oxygen.
 7. The device ofclaim 1 or 3, wherein the device is a chemical pump.
 8. The device ofclaim 1, wherein the conductive material is silver.
 9. The device ofclaim 1 or 3, wherein the electrolyte material is selected from thegroup consisting of yttria stabilized zirconia and yttria stabilizedbismuth oxide.
 10. The device of claim 1, wherein the electrode has aresistance which is less than or equal to about a fourth of theresistance of an electrode composed purely of the conductive material.11. A solid electrolyte based device, comprising a nanostructuredelectrolytic structure, prepared by aggregating nanoscale particles of aceramic electrolyte material to form the electrolytic structure; andsintering the electrolytic structure at a temperature sufficient tocause its densification, said temperature being less than 1000° C.,wherein the device is selected from the group consisting of an energystorage device, an energy generation device, a device for thermodynamicproperty measurements, a device for electrocatalysis, a device forelectrosynthesis, a solid state chemical pump, and a chemical sensor.12. The device of claim 11, wherein the energy generation device is abattery.
 13. The device of claim 11, wherein the energy generationdevice is a fuel cell.
 14. The device of claim 11, further comprising aporous nanocomposite solid electrode comprising a conductive materialand nanoscale particles of an electrolyte material and having a porefraction in the range of 15-50 percent.
 15. The device of claim 11,wherein the ceramic electrolyte material is stabilized bismuth oxide.